The evolving threats posed by concealed explosives or the intentional release of toxic chemicals demand new ways to detect these threats and protect the public. Typically, the techniques for identifying threat molecules involve ionizing a sample and then detecting whether the threat molecule (analyte) is present. The detection mechanisms include ion mobility spectrometry (IMS), differential mobility spectrometry (DMS), field asymmetric ion mobility spectrometry (FAIMS), and mass spectrometry (MS), all of which rely upon ionization of the analyte or a complex that includes the analyte. In fact, one of these techniques (IMS) is currently used in nearly every airport in the United States as a means to prevent concealed explosives from getting on aircraft.
Given the importance of these techniques to public safety, considerable effort has been devoted to develop the best means to collect the sample from the environment, present the sample to the instrument, and to also ionize it efficiently and if possible selectively, in order to provide the greatest detection capability.
In almost all instances, ionization is achieved selectively by performing the ionization under ambient-pressure conditions in the presence of an ionization reagent in a technique known as ambient-pressure ionization (API) (also sometimes called atmospheric-pressure chemical ionization). In API, the target analyte is drawn into a space containing both an ionization source and the ionization reagent, and ionization of the target molecule takes place through ion-molecule collisions. The ionization reagent is selected such that rapid achievement of charge equilibrium results in charge or proton transfer from the reagent to the target molecule.
Since many of the explosive and chemical threats have low vapor pressure and exist as traces of particulates or thin films on surfaces, the most common way to collect the sample requires a swipe or swab substrate which provides a physical mechanism to both collect and preconcentrate a sample taken from a surface of a suspect object for subsequent presentation to the ionization space of the detection instrument. The substrate media, which is called a “swipe,” can be thermally heated to desorb the target analyte into the vapor phase for subsequent ionization and detection. This methodology is currently used in fielded IMS systems that detect explosives, where detection relies on efficient collection and presentation of low-vapor analytes such as 2,4,6-trinitrotoluene (TNT), 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) and pentaerythritol tetranitrate (PETN) into the instrument, and use of ionization reagents that enhance the formation of negative ions via chloride adduction, such as methylene chloride.
In such explosive detection systems, the substrate is typically positioned in a thermal desorber located on the inlet side of the detection system. Thermal heating of the solid particles on the swipe induces a solid-to-vapor phase transition and releases the analyte molecules as a vapor, usually guided into the sensor inlet by a carrier gas, and the ionization reagent is introduced as a vapor within a separate carrier gas. Properties of commercially-available swipe media have been optimized over the years for increased efficiency of particle collection from surfaces (mechanical or electrostatic), efficient transfer and release of analyte into the chemical sensor, thermal stability, and low chemical background of the substrate.